Conventional polyetherester block copolymers comprising hard polyester segments, mainly comprises polybutylene terephthalate, and which are block polycondensed with soft polyether segments, mainly comprises poly(tetramethylene oxide)glycol, have not only a high thermoplasticity but, also, a proper softness, rubber-like elasticity, mechanical strength, resistance to chemicals and oils and thermal resistance. Therefore, it is well known that the conventional polyetherester block copolymer can be processed in the same manner as that usually applied to other thermoplastic polymers. Due to thermoplastic property, the polyetherester block copolymers are useful in the rubber and soft plastic industry fields.
Usually, the soft polyether segments in the conventional polyetherester block copolymers which are now commercially available, consist essentially of a poly(tetramethyleneoxide)glycol having a number average molecular weight less than about 1,500, preferably, of about 1,000. This limitation in the number average molecular weight of the poly(tetramethylene oxide)glycol is due to the fact that the soft polyether segments consisting of the poly(tetramethylene oxide)glycol are believed to be compatible with the hard polyester segments only when the poly(tetramethylene oxide)glycol has a number average molecular weight less than 1,500. The conventional poly(tetramethylene oxide)glycol which are now commercially available and has a number average molecular weight of 1,500 or more, exhibits a large variance (.alpha.) in molecular weight distribution of more than 1.70. The term "variance in molecular weight distribution", used herein, will be clarified in detail hereinafter. For example, the comercially available poly(tetramethylene oxide)glycol having a number average molecular weight of about 2,000, usually exhibits a molecular weight distribution variance of from 1.9 to 2.5.
When the poly(tetramethylene oxide)glycol having a number average molecular weight of 1,500 or more is used for forming the soft polyether segments, the resultant polyetherester block copolymer has a disadvantage in that the soft polyether segments exhibit a poor compatibility with the hard polyester segments. Accordingly, the soft polyether segments constitute their own phase separated from the phase consisting of the hard polyester segments not only in a solid state but, also, in a melted state of the polyetherester block copolymer. The melted state of the above-mentioned type of polyetherester block copolymer is opaque and exhibits a milky white color and pearly shine due to the phase separation of the polyether segments from the polyester segments. This phase separation causes the polyetherester block copolymer in the melted state to exhibit a poor fluidity and shaping property and the solidified copolymer to exhibit a poor mechanical strength and elasticity.
It is known that, in general, the larger the number average molecular weight of the poly(tetramethylene oxide)glycol, the higher the thermal properties, for example, melting point, crystallizing property, the mechanical strength at an elevated temperature and the injection molding property of the resultant polyetherester block copolymer. However, it is also known from Japanese Patent Application Laying-open No. 49-31795(1974) that the large number average molecular weight of the poly(tetramethylene oxide)glycol causes the resultant polyetherester block copolymer to exhibit a large degree of phase separation of the soft polyether segments from the hard polyester segments. This large degree of phase separation results in a poor uniformity in quality of the polyetherester block copolymer. The above-mentioned Japanese Patent Application Laying-open No. 49-31795 also discloses that, in order to prevent the phase separation of the soft polyether segments consisting of poly(tetramethylene oxide)glycol having a number average molecular weight of about 2,000, from the hard polyester segments, the poly(tetramethylene oxide)glycol segments should be contained in a proportion of 50% or more by weight, for example, 60% in the block copolymer. The large proportion of the poly(tetramethylene oxide)glycol segments naturally causes the proportion of the polyester segments to be small. The small proportion of the polyester segments naturally causes the polyetherester block copolymer to exhibit a poor crystallinity, mechanical strength, processability, and resistance to each of chemicals, heat and light.
Under the above-mentioned circumstances, in order to obtain the polyetherester block copolymer having a proper processability, the poly(tetramethylene oxide)glycol having a small number average molecular weight of about 1000 or less is used. The resultant polyetherester block copolymer has a relatively high compatibility of the soft polyether segments thereof with the hard polyester segments, and a relatively low melting point, crystallizing property and mechanical strength.